1. Field of the Invention
The present invention relates to a DC-DC converter which lowers a primary direct-current voltage to output a secondary side voltage, and in particular, relates to a DC-DC converter using current detection to generate a shutdown timing signal for an internal switching device.
2. Description of the Related Art
Miniaturization of apparatuses has become increasingly common year by year, and in power supply circuits, miniaturization and greater efficiency have been requested. In order to miniaturize a power supply circuit, it is important to make passive elements small, which is possible by making the switching frequency high. When a power supply circuit is configured with discrete semiconductor elements, it is impossible to operate at high speed due to inductance and the like coming from wiring in a wiring board and bonding wires. It is possible to reduce these influences significantly by configuring a DC-DC converter with a one-chip device.
Meanwhile, there is a DC-DC converter having a structure to detect current in an inductor provided on the output side and shut down (OFF) an internal switching device when the peak value of the current reaches a reference value (so-called current mode DC-DC converter). Here, when voltage fluctuation on the load side is fed to generate the reference value, the response as a DC-DC converter becomes favorable.
In the current mode DC-DC converter, current detection in the inductor is almost indispensable, but if a resistor is inserted to the inductor in series for this purpose, efficiency is badly affected. In other words, since current flowing to a load flows through this resistor, it is necessary to reduce the resistance value in order to limit loss. When the resistance value is made small, the generated voltage drop becomes small, which makes the current detection difficult. There is also parasitic inductance in the detected resistance, and this parasitic inductance causes more than a little error.
A related DC-DC converter is disclosed, for example, in the following Patent Document 1. In the disclosed DC-DC converter, current detection does not rely upon the resistor connected to the inductor in series, and therefore, in the sense of reduction of loss generated in the detection resistance, the disclosed converter is improved to some extent compared with an ordinary current mode DC-DC converter.
[Patent Document 1] JP-A 2001-245469 (KOKAI)